Question 1

What are the key new features introduced in Java 25?

Accepted Answer

Java 25 (LTS release, September 2025) features 18 JEPs focused on runtime performance and language simplification. Key highlights include finalized Scoped Values, Flexible Constructor Bodies, Compact Object Headers (reducing memory by 50%), Generational Shenandoah GC, Module Import Declarations, and instance main methods without class boilerplate.

Question 2

What is the difference between Java 25 LTS and previous LTS versions?

Accepted Answer

Java 25 builds on Java 21's modernization foundation by focusing on runtime optimization, delivering 30% faster startup, 50% smaller object headers, and production-ready generational GC. While Java 17→21 modernized the language with virtual threads and pattern matching, Java 21→25 optimizes the runtime and finalizes preview features like Scoped Values and Flexible Constructor Bodies.

Question 3

What preview features from previous versions have been finalized in Java 25?

Accepted Answer

Java 25 finalizes seven major features: Scoped Values (previewed in Java 21-24), Flexible Constructor Bodies (Java 22-24), Module Import Declarations (Java 23-24), Compact Source Files and Instance Main Methods (Java 21-24), Compact Object Headers (experimental in Java 24), Generational Shenandoah (experimental in Java 21-24), and the Key Derivation Function API (Java 24).

Question 4

What is pattern matching in Java and how has it evolved through Java 25?

Accepted Answer

Pattern matching allows you to test whether an object has a particular structure and extract components in a single operation. It evolved from simple instanceof checks in Java 16, to switch expressions in Java 21, and now includes unnamed patterns, record patterns, and primitive type patterns in Java 25.

Question 5

What are unnamed patterns and unnamed variables in Java 25?

Accepted Answer

Unnamed patterns use underscore `_` to match values without binding them to variables when you don't need the value. Each `_` is a separate unnamed pattern, preventing bugs by making it a compile-time error to reference them, and signaling intent to code reviewers.

Question 6

What are record classes and what problems do they solve?

Accepted Answer

Record classes are transparent carriers for immutable data that automatically generate constructors, accessors, equals(), hashCode(), and toString(). They solve the problem of boilerplate code in data-holding classes, reducing 30-50 lines of code to a single line while eliminating common bugs in manually written data classes.

Question 7

What are sealed classes and why were they introduced?

Accepted Answer

Sealed classes (introduced in Java 17) restrict which other classes can extend or implement them, providing a fixed set of permitted subclasses. They enable better domain modeling for closed sets of alternatives, improve API design by preventing unexpected subclasses, and enable exhaustive compile-time checking in pattern matching.

Question 8

What are virtual threads and how do they differ from platform threads?

Accepted Answer

Virtual threads are lightweight threads managed by the JVM rather than the OS, introduced in Java 21 and finalized from Project Loom. Unlike platform threads which are expensive (megabytes each) and limited to thousands, virtual threads are cheap (kilobytes) and can number in the millions, making synchronous, blocking I/O-bound code scalable without complex reactive patterns.

Question 9

What is structured concurrency and why is it important?

Accepted Answer

Structured concurrency treats multiple concurrent tasks as a single unit of work with a clear lifecycle, ensuring all subtasks complete before the parent completes. It eliminates thread leaks, provides reliable error propagation, and makes concurrent code more maintainable by preventing orphaned tasks and ensuring proper resource cleanup.

Question 10

What are scoped values and how do they differ from ThreadLocal?

Accepted Answer

Scoped values are an immutable, one-way data sharing mechanism finalized in Java 25 that provides better performance and safety than ThreadLocal. Unlike ThreadLocal's mutable variables that persist throughout a thread's lifetime, scoped values are immutable, have explicit bounded lifetimes, are automatically inherited by child threads, and offer significantly lower memory overhead.

Question 11

What are string templates and why were they introduced?

Accepted Answer

String templates (finalized in Java 23) enable embedded expressions in string literals with compile-time safety. They replace error-prone concatenation and String.format() patterns with safer, more readable syntax that prevents common security vulnerabilities like injection attacks through template processors like STR, FMT, or custom processors.

Question 12

What is the Foreign Function and Memory API?

Accepted Answer

The Foreign Function and Memory API (FFM API, finalized in Java 22) allows Java programs to interoperate with native code and manage off-heap memory safely and efficiently. It provides a modern replacement for JNI, offering better performance, safety, and ease of use through the Memory Segments API and Foreign Linker API.

Question 13

What are sequenced collections and why were they introduced?

Accepted Answer

Sequenced collections (introduced in Java 21) are collections with a defined encounter order and uniform APIs for accessing first and last elements. They solve the inconsistency problem across ordered collections by providing interfaces like SequencedCollection with methods like getFirst(), getLast(), and reversed() that work uniformly across List, Deque, and SortedSet.

Question 14

What is Generational ZGC and how does it improve upon ZGC?

Accepted Answer

Generational ZGC (production-ready in Java 21) adds generational collection support to ZGC, dividing the heap into young and old generations. This improves throughput by 5-25% and reduces memory overhead by focusing collection efforts on short-lived objects in the young generation, while maintaining ZGC's sub-millisecond pause times.

Question 15

What are Stream Gatherers and what problems do they solve?

Accepted Answer

Stream Gatherers (introduced in Java 22, finalized in Java 23) are custom intermediate stream operations that allow stateful transformations of stream elements. They solve the problem of complex operations that don't fit the traditional map/filter/reduce pattern, enabling sliding windows, batching, and custom aggregations without breaking the stream pipeline.

Java 25 LTS Overview(4 questions)

What are the key new features introduced in Java 25?

What is the difference between Java 25 LTS and previous LTS versions (Java 21, Java 17)?

What preview features from previous versions have been finalized in Java 25?

How do you migrate an existing application from Java 21 to Java 25?

Pattern Matching Enhancements(5 questions)

What is pattern matching in Java and how has it evolved through Java 25?

How do you use pattern matching for switch expressions in Java 25?

What are guarded patterns and how do you use them in switch statements?

What is the difference between type patterns and record patterns?

What are unnamed patterns and unnamed variables in Java 25?

Record Classes(5 questions)

What are record classes and what problems do they solve?

How do you create a compact constructor in a record?

What are the limitations of record classes?

How do you use records with pattern matching?

What are the best practices for designing immutable data with records?

Sealed Classes(4 questions)

What are sealed classes and why were they introduced?

What is the difference between sealed, non-sealed, and final modifiers?

How do sealed classes enable exhaustive pattern matching?

How do sealed classes improve API design?

Virtual Threads(6 questions)

What are virtual threads and how do they differ from platform threads?

How do you create and start a virtual thread in Java 25?

What is thread pinning and how do you avoid it?

How do you migrate existing thread pool-based code to virtual threads?

When should you NOT use virtual threads?

How do virtual threads handle blocking I/O operations?

Structured Concurrency(4 questions)

What is structured concurrency and why is it important?

How do you use StructuredTaskScope for concurrent tasks?

What is the difference between ShutdownOnSuccess and ShutdownOnFailure policies?

What are the benefits of structured concurrency over CompletableFuture?

Scoped Values(4 questions)

What are scoped values and how do they differ from ThreadLocal?

What is the performance advantage of scoped values over ThreadLocal?

How do scoped values work with virtual threads?

When should you use ScopedValue versus passing parameters?

String Templates(4 questions)

What are string templates and why were they introduced?

How do you use STR template processor for string interpolation?

What security benefits do string templates provide?

How do string templates prevent SQL injection attacks?

Foreign Function and Memory API(4 questions)

What is the Foreign Function and Memory API?

How do you allocate off-heap memory using the Memory API?

How do you call native C functions from Java using the Linker?

How does the Foreign Function API compare to JNI?

Sequenced Collections(3 questions)

What are sequenced collections and why were they introduced?

How do you use getFirst(), getLast(), and reversed() methods?

What existing collections implement SequencedCollection?

Generational ZGC(3 questions)

What is Generational ZGC and how does it improve upon ZGC?

What is the difference between ZGC, G1GC, and Generational ZGC?

When should you choose Generational ZGC over other collectors?

Stream Gatherers(3 questions)

What are Stream Gatherers and what problems do they solve?

How do gatherers differ from collectors?

What is the windowFixed() gatherer and when would you use it?

Best Practices and Migration(4 questions)

What are the best practices for writing idiomatic Java 25 code?

How do you decide between records, classes, and sealed hierarchies?

What is the recommended approach for concurrent programming in Java 25?

What are the common migration pitfalls from Java 17/21 to Java 25?